1. Statement of the Technical Field
The inventive arrangements relate to electronic article surveillance systems, and more particularly to synchronization of two or more electronic article surveillance systems which have the potential to interfere with one another.
2. Description of the Related Art
Pulsed magnetic EAS systems operate by generating a short burst of magnetic flux in the vicinity of a transmitter antenna. This pulsed field stimulates a particular type of magnetic label or marker, whose characteristics are such that it is resonant at the operating frequency of the system. The marker absorbs energy from the field and begins to vibrate at the transmitter frequency. This is known as the marker's forced response. When the transmitter stops abruptly, the marker continues to ring down at a frequency which is at, or very near the system's operating frequency. This ring down frequency is known as the marker's natural frequency. The vicinity of the transmitter antenna in which the response can be forced is the interrogation zone of the EAS system.
The magnetic marker is constructed such that when the marker rings down, the marker produces a weak magnetic field, alternating at the marker's natural frequency. The EAS system's receiver antenna, which may be located either within its own enclosure or within the same enclosure as the transmitter antenna, receives the marker's ring down signal. The EAS system processes the marker's unique signature to distinguish the marker from other electromagnetic sources and/or noise which may also be present in the interrogation zone. A validation process must therefore be initiated and completed before an alarm sequence can be reliably generated to indicate the marker's presence within the interrogation zone.
The validation process is time-critical. The transmitter and receiver gating must occur in sequence and at predictable times. Typically, the gating sequence starts with the transmitter burst starting with a synchronizing source, such as the local power line's zero crossing. The receiver window opens at some predetermined time after the same zero crossing.
In a three phase power system, power lines within a building can have individual zero crossings at 0°, 120° or 240° with respect to each other. Accordingly, different EAS units plugged into different electrical outlets may detect a zero crossing at either the 0°, 120° or 240° point in the line frequency's period. In this way, a first EAS system, referred to as system A, can have a different zero crossing reference time as compared to a nearby EAS system, referred to as system B.
In order to compare received signals to background noise, separate noise averages are continuously sampled, computed and stored as part of a signal processing algorithm. This is commonly done by operating the EAS systems at 1.5 times the power line frequency, 90 Hz for a 60 Hz line frequency or 75 Hz for a 50 Hz line frequency, and alternating the interpretation of each successive phase. More particularly, if phase A is a transmit phase (the receiver window is preceded by a transmitter burst), phase B will be a noise check phase (the receiver window was not preceded by a transmitter burst), phase C will be a transmit phase, phase A will be a noise check phase, and so on.
EAS systems operating in proximity to each other must be synchronized in some way to prevent them from causing interference with one another. Previous implementations of pulsed magnetic EAS systems have utilized various approaches to ensure synchronization. Some systems are manually synchronized by a technician, and rely on a power line frequency zero crossing as a reference time. Another approach is more automated but requires a wired connection between respective system processor boards of the multiple EAS systems. Other systems utilize wireless synchronization methods. These wireless systems can involve wireless communications among two or more EAS systems that are designed to accommodate such wireless synchronization methods. For example, one such wireless system is disclosed in U.S. Pat. No. 6,201,469 to Balch, et al.
A plurality of EAS systems operating in proximity to one another can be synchronized by the various methods described above, provided that (1) a technician has authorized access to all of the EAS systems which are to be synchronized and/or (2) each of the EAS system is specifically designed to participate in a particular automated synchronization method (wired or wireless) which is being used. But there are some instances where one or more of the EAS systems in a proximate area are not designed to utilize a particular automated synchronization method or are not under the control of a technician who is attempting to manually synchronize operation of two or more EAS systems. For example, this can occur when a plurality of EAS system are made by different manufacturers who utilize different automatic synchronization schemes. Alternatively, this can also occur when EAS units are operated or maintained by a different entities and one of the EAS units has been improperly synchronized by a technician with inadequate training or indifference to the interference problem. EAS systems of this kind can be thought of as non-cooperative EAS systems.